Physical Metallurgy of Thermomechanical Treatment of Structural Steels

182 Pages·1.698 MB·English

by  Mazanec K., Mazancova E.

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title : Physical Metallurgy of Thermomechanical Treatment of
Structural Steels
author : Mazanec, Karel.; Mazancova, E.
publisher : Cambridge International Science Publishing
isbn10 | asin : 1898326436
print isbn13 : 9781898326434
ebook isbn13 : 9780585237466
language : English
subject Physical metallurgy, Steel--Metallurgy.
publication date : 1997
lcc : TN693P49 1997eb
ddc : 669/.96142
subject : Physical metallurgy, Steel--Metallurgy.
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Page iii
Physical Metallurgy of Thermomechanical Treatment of Structural Steels
Karel Mazanec and Eva Mazancová

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Published by
Cambridge International Science Publishing
7 Meadow Walk, Great Abington, Cambridge CB1 6AZ, UK
http://www.demon.co.uk/cambsci/homepage.htm
First published December 1997
©K Mazanec and E Mazancová
©Cambridge International Science Publishing
Conditions of sale
All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means,
electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without
permission in writing from the publisher
British Library Cataloguing in Publication Data
A catalogue record for this book is available from the British Library
ISBN 1 898326436
Translated by V E Riecansky
Production Irina Stupak
Printed by Transtech Printers Ltd, London, England

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About the Authors
Prof Ing Karel Mazanec, DrSc graduated from the Technical University in Ostrava in 1949. Since then, he has worked
in a number of leading posts, including the Vitkovice Iron Works Research Institute in the area of the development of
new types of steel for power engineering, research into the thermomechanical treatment of structural and high-strength
steels, their weldability and brittle fracture resistance. In 1962, he defended his DrSc dissertation on 'Delayed fracture in
martensite'.
Prof Mazanec has published more than 350 studies in many leading journals and conference proceedings. In 1965 he
became a professor at the Faculty of Metallurgy of the Technical University in Ostrava and later Dean of the Faculty. In
1983 he was elected a member of the Czechoslovak Academy of Sciences. He is also a honorary member of the French
Metallurgical Society. At present, is a Professor Emeritus at the Technical University of Ostrava where he lectures and
works in the research of unconventional methods of heat treatment of steel and physical-metallurgical problems of
strengthening and increase of fracture resistance. Recently, he has been paying attention to physical-metallurgical
problems of thermoelastic martensite in shape memory TiNi alloys.
Ing Eva Mazancová, CSc, graduated from the Technical University of Ostrava in 1975 and started her career at the
Research and Testing Institute of the New Iron Works in Ostrava where she now works in the structural and
microfractographic analysis of structural steels, microanalysis of nonmetallic inclusions and research into segregation
phenomena, including their effect of the properties of steel.
In 1981, Eva Mazancová defended her PhD dissertation on 'Thermomechanical treatment of structural steels' in which
she examined the application of thermomechanical treatment of microalloy steels in seamless tube production. She has
also worked on problems of hydrogen and sulphide brittleness of materials for oil industry, the effect of chemical
constitution and microstructure on corrosion cracking; problems associated with optimization of the heat treatment of
oil pipes with higher strength and their physical-metallurgical characteristics also attracted her attention.
Her current interests include problems of microfractographic analysis of structural steels and evaluation of relationships
between microstructure, fracture parameters and cleanness of steel. She is also involved in evaluating the
microcleanness of continually cast steels and its effect on the properties of structural steels after various heat treatments.
During her scientific and research activity, Eva Mazancová has published 105 studies in leading journals and
conferences, both home and abroad.

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Contents
1 1
Introduction
2 3
Main Physical Metallurgy Characteristics of Thermomechanical Treatment
5
2.1 Sources of Hardening the Metallic Matrix
20
2.2 Analysis of Physical Metallurgy Characteristics of the Initial Austenitic Matrix Taking into Account
the Subsequent Heat Treatment
34
2.3 Influence of the State of the Initial Austenitic Matrix on Phase Transitions in Thermomechanical
Treatment
39
2.4 Physical Metallurgy of the Ferritic Phase Transformation in Work-Hardened Austenite
3 56
Thermomechanical Treatment of High-Strength Martensitic Steels
61
3.1 Effect of Thermomechanical Treatment on Properties
75
3.2 Effect of Thermomechanical Treatment on the Modification of Microstructural Characteristics
81
3.3 Evaluation of the Mechanical and Metallugical Properties of High-Strength Martensitic Steels after
HTMT
82
3.4 Summary of the Achieved Results and Their Physical Metallurgy Analysis
4 93
Thermomechanical Treatment and Controlled Rolling of Carbon and Low-Alloy Steels
93
4.1 Application of HTMT in Strengthening the Matrix of Steels for Wider Technical Application and
Determination of the Relationships between the Initial Structure and the Resultant Physical Metallurgy
Properties of Products of Austenite Decomposition
101
4.2 Technical and Technological Characteristics of Controlled Rolling
103
4.2.1 Deformation in the Austenite Range Associated with Recrystallization
104
4.2.2 Deformation in the Region of Restricted Recrystallization
108
4.2.3 Deformation in the AusteniteFerrite (Two-Phase) Region
110
4.3 Application of Various Variants of Accelerated Cooling after Controlled Rolling
119
4.4 A Model of Predicting Changes in the Microstructure in Controlled Rolling and Accelerated Cooling
of Structural Steels for Wider Technical Application
120
4.4.1 Perspective Technical and Technological Solutions
122
4.5 Effect of Controlled Rolling and Accelerated Cooling on Reducing the Susceptibility to Hydrogen
Embrittlement
131
4.6 Technical and Technological Comments Regarding Application of Controlled Rolling and
Accelerated Cooling
5 134
Conclusions
References 137
Index 141

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1
Introduction
The development of science and technology imposes new requirements on obtaining higher applied properties of
metallic materials, especially higher strength and ductility properties. The main methods based on complex or higher
alloying or on the development of metallic materials with new constitutions usually enable higher material and technical
properties to be obtained, but this is achieved with higher or more extensive requirements on alloying elements. This
increases the production cost. Despite the fact that in many cases this is the only possible approach that can be used, on
a wider scale higher applied material and technical properties can be achieved in various types of metallic systems by
other methods usually based on the application of new, unconventional heat treatment technologies. These solutions
form suitable conditions for the maximum utilization of the processing properties of appropriate types of metallic
materials.
This has resulted in the development of a large number of unconventional heat treatment methods of metallic materials,
some of which are based on using highly complicated technical and processing variants, as discussed in, for example,
Ref.1. According to the currently available experience, different variants of thermomechanical treatment (TMT)
represent one of the most efficient methods of unconventional heat treatment. This method of unconventional heat
treatment can be used widely not only for steels but also in heat treatment of, for example, alloys of aluminium,
titanium, nickel, etc.2
In this book, the authors pay special attention to analysis of the physical metallurgy parameters and structuralmaterial
characteristics obtained in structural steels or iron-based alloys using various types of TMT. This method makes it
possible to obtain higher applied properties of these materials, without any large changes in their chemical constitution.
From the viewpoint of developing this heat treatment method, the proposed solution will include not only an analysis of
basic variants of TMT in the region of stable and unstable austenite in high-strength martensitic steels and alloys but
also the problems of application of TMT in optimizing the properties of low-alloy or carbon structural steels in which
the austenite

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to martensite transition does not take place during the phase transformation. Consequently, for these steel types we shall
also discuss selective problems of the physical metallurgy of controlled rolling or the characteristics of accelerated
cooling, which are closely linked with the problem of the processes of TMT of structural steels.
The main feature of all these variants of TMT is that the plastic deformation of the initial austenitic matrix and the
formation of its controlled structural and mechanical state with subsequent heat treatment, phase transformation, are
linked. This link makes it possible to utilize the hardening or refining of the grains of the initial austenitic matrix and
also the structural modification of the products of the phase transformation of austenite associated with the modification
of structural and substructural characteristics of the austenitic matrix leading to a higher level of the set of the
mechanical and metallurgical properties of various types of structural steel processed by this treatment.
The book summarizes the results obtained in extensive research of the hardening characteristics of metallic materials
(structural steels), with special attention given to the problems of the physical metallurgy nature of hardening of
martensitic steels and control-rolled structural steels or structural steels subjected to subsequent accelerated cooling and
used for wider technical applications. Taking this analysis into account, selected problems, whose solution leads to
obtaining optimized technical and technological conditions of processing these types of steel by the selected
unconventional heat treatment method, will also be discussed.
The aim of this book is to present a detailed analysis of the main parameters of unconventional treatment of structural
steels resulting in higher strength and ductility properties of these materials, including higher utility properties. This
makes it possible to create suitable conditions for obtaining a higher level of safety and reliability of structures or
structural components produced from materials treated in this manner. The main technical and technological
characteristics of TMT of structural steels of different chemical composition and the conditions of their optimum use
will also be determined.

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2
Main Physical Metallurgy Characteristics of Thermomechanical Treatment
Information on the controlling role of the structure and the conditions of influencing the so-called structure-sensitive
characteristics of metallic materials has resulted in conclusions according to which the considered structure-sensitive
properties are strongly affected by the defectiveness of the matrix. It is well known that lattice defects, present in the
initial matrix, also strongly influence by both the mechanism and kinetics of phase transformations in heat treatment, i.e.
the characteristics of the resultant structure and the final mechanicalmetallurgical properties of this type of metallic
material. This shows that the aim of the solution must be to obtain both the favourable density of structural defects,
namely dislocations, and their arrangement in the metallic matrix in various stages of TMT in order to obtain, in
controlling the appropriate processes, the required structure and, consequently, the optimum level of the final
mechanical and metallurgical characteristics.
Plastic deformation and subsequent recovery processes in the parent metallic matrix are the main metallurgical
processes enabling control of the dislocation density in the metallic matrix. These considerations then lead logically to a
simple conclusion, according to which the most efficient method is to combine efficiently the appropriate physical
metallurgy characteristics of phase transformations and plastic deformation (including control of the subsequent or
simultaneous development of recovery and recrystallization processes) and the formation of a single technological unit.
TMT is then a process resulting in the detailed coupling of controlled modification of the structure in deformation of the
initial austenitic matrix and its decomposition in heat treatment.
As indicated by previous studies concerned with the analysis of TMT conditions of various types of steels, this process
must be regarded as a set of partial processes taking place during heating, deformation and cooling of metallic materials.
This results in the formation of a structure enabling properties to be obtained similar to those which can be achieved in,
for example, materials at a higher density and specific arrangement of lattice defects gen-

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erated in plastic deformation in the metallic matrix prior to the final phase transformation.1,2
Initial studies in the area of research into the physical metallurgy and technical and technological characteristics of TMT
in, for example, structural steels, were aimed at determining the conditions for obtaining the maximum level of strength
whilst maintaining the required toughness level.1,3 Therefore, special attention was given to two main TMT variants:
evaluation of the technical and technological parameters of TMT in the stable austenite range (high-temperature
thermomechanical treatment, HTMT), and thermomechanical treatment in the unstable austenite range (low-temperature
thermomechanical treatment, LTMT). In the former case, plastic deformation is carried out in stable austenite, whereas
in the latter case plastic deformation takes place in unstable austenite, usually connected with subsequent quenching to
martensite1 leading to a large increase of the mechanical and metallurgical properties of alloyed high-strength
martensitic steels. During the subsequent development of TMT technologies, the coupling of plastic deformation with
subsequent heat treatment was also extended to carbon and low-alloy steels, where the product of final heat treatment is
usually the formation of a ferriticbainitic, bainitic or bainiticmartensitic structure.4 In all these cases, this is achieved by
the rapid cooling of appropriate types of structural steels with the defined parameters of the initial austenitic structure, e.
g. in a quenching press.1
In addition, in the further development of unconventional methods of heat treatment of structural steels, as a part of a
wider concept of technical and technological variants of TMT, work is being carried out on technical and technological
variants of controlled rolling or the superposition effect of accelerated cooling. This method is used mainly in low-
carbon or microalloy steels for wider technical applications alloyed with, for example, niobium, vanadium, titanium, or
with several microalloying additions. Usually, the phase transformation of the austenitic matrix is accompanied by the
formation of non-martensitic products, with a special position in optimizing the resultant properties played by the
optimum form of the resultant ferrite grains. The addition of microalloying elements, together with the modification of
the reduction schedule in forming and the selection of holding time between passes (control of the temperature of
completion of forming and the development of recovery processes in the parent metallic matrix austenite) has a
beneficial effect on the resultant austenite grain size and the development of recrystallization processes (static and
dynamic recrystallization) and the extent of strain-induced precipitation of carbides or carbonitrides of microalloying
additions.5

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